Upon arrival checked in with Mrs. Livermore and located both systems, system one controls first and second floors while the other system controls the third and fourth floors. The second system was not running upon arrival. Went into the attic and found in line float switch had tripped. Re worked drain line and found water was being forced to secondary drain port due to the force of the air inside of the supply plenum, I explained to homeowner that with one zone open the blower motor trims back its speed but with both zones open it is too much pressure inside of the supply plenum and ductwork is undersized. To combat this issue recommend adding a static bypass damper to open up when both zones are calling to alleviate the static pressure in the supply plenum. Also found widespread organic growth inside of the supply plenum. Recommend replacing with new supply plenum to have fresh insulation without any growth. This is being caused by airflow issues, improper sealing of the supply p